Fix main public interface name not always be eth0.
Fix main public interface name not always be eth0.

I found that in Xenial. The main public interface name is not
always be eth0. So role bootstrap-host have to change from eth0 to
ansible_default_ipv4.interface to get the correct default
public interface name

Change-Id: Iab26e48010b57240110e5235dd059622dd8057cd
Closes-Bug: #1647107

bunchc/openstack-ansible,openstack/openstack-ansible,stackforge/os-ansible-deployment,os-cloud/os-ansible-deployment,robb-romans/openstack-ansible,os-cloud/os-ansible-deployment,weezer/openstack-ansible,BjoernT/os-ansible-deployment,yanyao/openstack-ansible,BjoernT/openstack-ansible,cfarquhar/openstack-ansible,BjoernT/openstack-ansible,yanyao/openstack-ansible,stackforge/os-ansible-deployment,cloudnull/os-ansible-deployment,BjoernT/os-ansible-deployment,openstack/openstack-ansible,cloudnull/os-ansible-deployment,bunchc/openstack-ansible,weezer/openstack-ansible,robb-romans/openstack-ansible,cfarquhar/openstack-ansible,BjoernT/os-ansible-deployment